As a laser (or other light source) and sensing lens are moved radially (by an optical pick-up unit (“OPU”)) with respect to a disc in a disc drive, the distance between the surface of the disc and the lens (hereinafter referred to as a Z-distance) is substantially constant. However, various imperfections may arise over the surface of the disc and/or the shape of the disc may vary. For example, the disc may be subject to “potato chipping” (i.e., where the disc bends along a diameter) or “cupping” (i.e., where the outer edge of the disc are generally planar but not coplanar with the center of the disc). Previous methods and apparatuses have addressed these imperfections such that the lens remains substantially at a Z-distance of optimum focus with respect to each portion of the disc. The Z-distance of optimum focus will correspond to the Z-distance at which a maximum amount of data can be written to a disc or read from it. There may, however, be other applications in which the Z-distance of optimum focus may not be ideal. Accordingly, there is a need to know the gain corresponding to optimum focus at each location on a disc.